VM DO limit downloads
Virgin Media definitely DO limit downloads with a speed cap during peak hours. I've been with VM for 8 years and on the 20 meg package you get limited to 500KBPS once you've DL'ed 2GB. Peak hours seem to me to be 4PM to 12AM. Don't bother trying to talk to their tech support about it as they don't even know what traffic shaping is let alone how they are applying it or download throttles.
Saying that, the VM 10meg package is the one to go for as they can actually sustain 10meg (barring the throttling). With the 20meg you'll rarely see it and performance can and does drop as low as 70Kbps (at any time). With the 10 meg it stays at 10 meg consistently (barring throttling).